Wavelength modulation



5 Sept. 15, 194

c. N. KIM BALL WAVELENGTH MODULATION 2 Sheets-Sheet 1 Filed Oct. 2, 1940 F E CARRIE/Z CARR/ ER W4 VE 2 E CARR/ER E CARR/L512 CARR/ER WA VE INVENTOR CHARLES N. KIMBALL ATTORNEY .C. "N. KIMBALL WAVELENGTH MODULATION Sepl. 15,1942

Filed Oct; 2, 1940 2 Shets-Sheet 2 INVENTOR CHARLES rigs/AL BY ATTORNEY v phase.

Patented Sept. 15, 1942 signor to Radio Corporation of America, a corporation of Delaware Application October 2, 1940, 'Serial No. 359,314

. 14 Claims.

This application concerns a method of and means for modulating the wave length of wave energy in accordance with signals. The particular means I have provided for modulating the wave length of wave energy is essentially a phase modulator. As will be seen by those skilled in .5 the art once they understand my system, the

same may also be used to produce frequency modulation of wave energy.

An object of my invention is to provide a new and improved wave length modulation system which is simple in nature, reliable in operation, and which requires a small'amount of circuit elements and tubes to carry out the modulationprocess.

Another object of my invention is to attain the object set forth in the preceding paragraph and to, at the same time, provide a system wherein the wave length modulations are linear with respect to the amplitudes of the modulating potentials. I

In its broadest aspect my system includes a wave length modulation system arranged in accordance with my invention;

Figs. 2 and Bare respectively a basic circuit used to show the manner in which the phase displaced voltages are developed in the tube of Fig. 1 and a Vector diagram also used to show how the voltages developed in my novel system, one of which is of variable phase, cooperate to produce a resultant phase modulated output voltage;

Fig. 4 illustrates a modification of Fig. 1, while Fig. 5 illustrates the manner in which a balanced modulation system, in accordance with my discharge device having electrodes including a;

control electrode, a cathode, and an electron collecting electrode withthe, cathode raised above ground or equivalent alternating-current potential by a reactance. Voltages of carrier wave frequency are impressed across the grid-to-cathode tube space and the reactance in series. This causes alternating current to flow in this series circuit. The current flowing in the series circuit leads or lags the voltage in the series path depending on whether the reactance is capacitive or inductive. As a consequence, an alternating-current voltage which is displaced in phase relative to the original voltage developedacross the series circuit is developed between the grid and cathode. By modulating the mutual conductance of the device the phase of the voltage appearing across the cathode reactance in the series circuit is varied. The voltage developed between the grid and cathode is correspondingly modulated in This phase variation is linear with respect to variations in mutual conductance of the device and the latter are made linear with respedt to modulation potential range.

Preferably, the carrier voltage worked with is of relatively low frequency, say, for example, 100 kilocycles. The modulated carrier is then' irequency multiplied to the required extent, limited, if desired, amplified and used to convey the modulation potentials. v

In describing my invention in detail, reference will be made to the attached drawings wherein:

Fig. 1 illustrates the essential elements of a.

invention, is arranged.

A simplified circuit arrangement of my modulator is shown in Fig. 1. The basic circuit comprises a pentode H] with a reactive load Ck in its cathode circuit. The load may be either inductive or capacitive, depending upon the desired direction of carrier phase shift. The basic circuit is drawn below.

Tube I0 is a pentode, with mutual conductance gm and plate resistance no so high that its plate alternating current zp equals its mutual con ductance, gm times its grid alternating excitation voltage Egk.

As stated above, the object of the circuit is to cause the phase of the grid-to-cathode voltage Egk to vary in accordance with the amplitude of the modulation voltage E mod. This variation in phase acts to vary the phase of the plate current in the tube relative to its nominal unmodulated phase. 1 i

The tube I0 has its grid I2 excited by carrier wave voltage from source 8' through coupling condenser 9 and also by modulation voltage from source 6 through radio-frequency choke 1.

The diagram shows E mod (an audio voltage) and E carrier (a radio-frequency voltage) applied to a common grid through appropriate chokes and filters, but other types of connection, i. e'., multi-grid tubes, may be employed Neglecting the direct-current considerations, the basic circuit is as illustrated i: Fig. 1 and in Fig. 2 which is a simplified circuit similar to that of Fig. 1.

I ;=E zgm (Basic action of the pentode.)

Ek=IpzlC where Z1: is the impedance in the cathode load. (Impedance drop in Zr.)

Egk=E carrier-Er (The cathode potential is subtracted from the potential impressed on the grid to get Egk.)

From these three equations we get Egk=E carrier-:(1+Zkgm) In the figures jwC,. therefore E carrier lk W V m and Egk leads E carrier by waves so produced are utilized for signalling purposes directly or after multiplication, amplification, and amplitude limiting.

If the cathode load had been an inductive reactance of Law, the value of Egk would have been E carrier w/lw k and the vector representing Egk would lag that representing E carrier by =tanwLkgm.

Considering the capacitance loading case, the vector diagram of Fig. 3 shows the action of E3]: is in the first quadrant with the vector E carrier as shown.

In these single-sided circuits (or in any phase modulator) it is essential that vary substantially linearly with the instantaneous value of E mod. Hence is restricted to values for which tan within, say, 5%. Thus, 25 may be taken as top value for for acceptable distortion. Tan 25=.466.

To examine the effect of a variation of gm (due to E mod) on the amplitude of the plate current resulting from a shift in the phase of Exit it is necessary merely to note that Ip=gmEge Hence, where a capacitive reactance Thus, where =tan 5 within 5% and 4 is in radians Equation A may be approximated by the following equation I E carrier C qs It is obvious that if angle is varied between 25 and 0 that the 0 point is attained only by reducing the mutual conductance gm to zero, which results in zero alternating plate current 1 hence the variation in angle must be restricted to a range resulting in acceptable amplitude modulation. Since E carrier and Cu are considered constant we need to examine the variation in In with at =25 or .436 radian, using exact Formula A.

I,=E carrier Ca (.436) At =10 Ip=E carrier Cm (.176) r v Hence, if the initial unmodulated value of were set so that the value of 4 (for zero E mod) were approximately the modulating voltage could be used to swing gm above and below this value, to cause maximum phase shifter 25 and a minimum of 10 resulting in approximately 39+% amplitude modulation, 1. e.,

This amplitude modulation may be clipped of! in a subsequent limiter stage.

The useful region of phase variation lies between the values shown above, unless one wishes to get about 5 more degrees phase modulation (i. e., a minimum value of =5 degrees) in which case the resultant amplitude modulation is greater.

A similar analysis can be made for the case of inductive cathode loading, and the results are the same if Lkw is substituted for L C en in the preceding development.

This is the basic scheme of the invention, i. e., to cause Egk to vary its phase relation with respect to its unmodulated position. It can be shown to do so without substantial variation in amplitude. Due, however, to the dependence of the amplitude 29 (alternating current component) on the value of gm, as varied by E mod, the use of ip in this phase variation tube restricts the value of phase shift obtainable by the system. Any reduction in in which can be attained will obviously reduce the undesired amplitude modulation and permit use of larger phase deviations.

Hence, to improve this situation, consider Fig. 1 modified as shown in Fig. 4, to which is added a second tube 20 having its grid 2% and cathode 25 coupled by tuned circuits 28 and 30 to the grid l2 and cathode ll of tube I0.

A high impedance tuned circuit, resonant at the constant frequency of E carrier is connected l0. Its impedance is high compared to -carrier voltage on the grid .of tube 2|! E z=gmz times a constant K times the voltage Egki between the grid and cathode of tube l0 between grid 12 and cathode H of the first tube 1 Z 18 Egk=E carrier when gm1=0, and, when I L{ C .45 (for =25 phase deviation in Egk fromE carrier) E carrier E',;, =.91 E carrier an amplitude modulation of only 9%.

Thus, in the modification of Fig. 4, the first tube is used only to efiect a phase change of the second tubes grid voltage. The plate current variations in the first tube are notused to supply any utilization circuit. Note that the full capabilities of the scheme are exploited by driving the first tube from zero bias (or even positive values of E on the modulating grid to cut-off, to get maximum phase deviation.

As shown above, the system is essentially single-sided, but phase deviations on both sides of the quiescent unmodulated phase are obtainable by using push-pull tubes for the phase changers, one with a capacitor'in the cathode and the other with. an inductive cathode load. In Fig. 5, I have illustrated a circuit using tubes 40 and 60 to phase deviate the carrier about a medium phase. In Fig. 5, the. second set of tubes 80 and I are connected in conventional pentode style.

In Fig. 5, the control grids M and GI of tubes 40 and 60 respectively are coupled by blocking condenser 43 and resistance 45 to a source of carrier wave voltages 46 to be excited thereby substantially in phase. The cathode 41 of tube 40 is bias resistor 65 and a carrier wave bypass con- I denser BC. The grids 4| and GI are connected in push-pull relation by the secondary winding oia transformer 66 the primary winding of which is connected to a source of modulating pc- I tential 6. A negative bias is supplied to the grids I and GI by source II. The remaining grids in the tubes 40 and 60 are connected as shown and the connections are believed self-explanatory.- The direct-current circuit of cathode 41 comprises the radio-frequency. choke and bias 'resistor 6 9 in series between the saidcathode and ground. The grid 41 is connected by blocking condenser I0 to a tuned circuit II including an inductance which forms the primary winding of transformer". The secondary winding of this transformer is included in a tuned circuit I3 connected to the grid 8| of tube 80. A similar blocking condenser I5, tuned circuits 16 and 11 of transformer 18 couples the grid GI and cathode 61 to the grid IOIof the tube I00. Thecathodes 03 and I03 of tubes 80 and I00 are connected to ground by biasing resistors '85 and I05.

These biasing resistances are shunted by radiofrequency bypass condensers BC. At this point it is noted that the screen electrodes of tubes 40 and 00 are also connected to the cathodes by bypass condensers BC. The anodes 81 and I0! of tubes and I00 are tied together and connected to a tank circuit I08 from which the phase modulated voltages are derived.

The screen electrodes in tubes 80 and I00 are connected to positive potential sources. It is thought unnecessary to further describe this stage comprising tubes 80 and I00. The screen electrodes of tubes 40 and 60 are connected by resistances 49 and III to a positive potential source while the .anodes of these tubes are likewise connected to a positive potential source.

The first pair of tubes are driven in push-pull by the modulation potentials from 6 and allel by carrier wave from 46;

in parat the input carrier frequency if the tubes 40 and 60 have similar mutual conductance characteristics gm. The tubes 40 and 60 are degenerative for modulation potentials (E mod), due to theirbias resistors 65 and 80 being unbypassed for audio frequencies. The tubes 40 and 50 are so biased that the combined push-pull mutual conductance times grid modulation potential (gm vs Eg mod) characteristic is essentially linear over the operating range, i. e., each bias prob! ably quite near cut-off.

The tube 40 causes its grid-to-cathode carrier voltage Egk to lead its unmodulated phase position when its mutual conductance cm is increased. The tube 60 causes a phase lag in the carrier voltage of an equal amount during alternate half cycles of the modulating voltage wave.

The transformers 'I2 and I8 feeding the second pair of tubes 80 and I00 may be arranged for voltage step-up so that the grids of the tubes 80 and I00 are overloaded, to produce harmonic currents in the plate circuits thereof. These tubes may then be used for frequency or phase deviation multiplication. Of course, they may operate as amplifiers in which case multiplication may take place in latter stages. When used as amplifiers only, the tubes 80 and I00 are not overloaded. The tuned'plate load I08 may be placed either' between plates 81 and I01, or between the plates. 81 and I0! in parallel and B+, depending upon the desired order of multithe winding of I08 direct current is supplied to v a tap on the said winding.

Maximum phase deviation in. either tube should be restricted to about 25-30, i. e., approximately .45 radian for a frequency deviation of nKc (ultimate) the maximum phase deviation is f. EE mlt f f radians where f is th cycles.

audio frequency in kilo- For a system with 30 cycles as the lowest tone and kilocycle maximum deviation (200 kilocycle band) gg =3333 radians Thus, with this scheme, if about .5 radian deviation is produced in e modulating stage, an

ultimate frequency multiplication of about 6600 is required. For narrow band systems this is reduced in proportion to the reduction in band width.

The necessary signal frequency multiplication may be accomplished by doubling or tripling several times in subsequent over-biased stages, etc. This is a familiar feature of all phase modulation systems.

If constant amplitude audio voltage is supplied to the modulator for all the audio frequencies involved, the output of the device is a phase modulated signal, 1. e., the phase displacement of the carrier is independent of the audio fre quencies employed. A frequency modulated signal with uniform frequency deviation for all audio frequencies is obtained by first applying to the modulator, an audio voltage whose amplitude varies inversely with audio frequency.

The source frequency, that is the frequency of E carrier, will be in the vicinity of 100 to 300 kilocycles if the ultimate carrier frequency is approximately 40 megacycles. The exact value of the source frequency will depend upon the value of the carrier frequency employed and upon the required ultimate frequency deviation.

What is claimed is:

1. In a wave length modulator, an electron discharge device having electrodes including a grid and a cathode, a carrier frequency reactance in the cathode circuit of said device, means for developing a voltage of carrier wave frequency to be modulated across the impedance between said grid and cathode and said reactance in series whereby voltages of said frequency appear in said series circuit, and a phase displaced voltage of said frequency is developed between said grid and cathode, means for varying the mutual conductance of said device at signal frequency to thereby correspondingly vary the phase of said last voltage, and means for utilizing said last voltage.

2. In a wave length modulator, an electron discharge device having electrodes including a grid and a cathode, a capacitive carrier frequency reactance in the cathode circuit of said device, means for developing a voltage of carrier wave frequency to be modulated across the impedance between said grid and cathode and said reactance in series whereby voltages of said frequency appear in said series circuit, and a voltage of said frequency and of a phase which leads the phase of said first voltage is developed between said grid and cathode, means for varying the mutual conductance of said device at signal frequency to thereby correspondingly vary the phase of said last voltage, and means for utilizing said last voltage.

3. In a wave length modulator, an electron discharge device having electrodes including a grid and a cathode, a carrier frequency inductive reactance in the cathode circuit of said device, means for developing a voltage of carrier wave frequency to be modulated across the impedance between said grid and cathode and said reactance in series whereby voltages of said frequency appear in said series circuit, and a voltage of said frequency and of a phase which lags the phase of said first voltage is developed between said grid and cathode, means for varying the mutual conductance of said device at signal frequency to thereby correspondingly vary the phase of said last voltage, and means for utilizing said last voltage.

4. A. modulator as recited in claim 1 wherein said device is of the pentode type and of high plate resistance.

5. A modulator as recited in claim 2 wherein said device is of the pentode type having a high plate resistance. r

6. A modulator as recited in claim 3 wherein said device is a pentode of high plate resistance.

'7. In a wave length modulation system, an electron discharge device having electrodes including a grid and a cathode, a reactance in the cathode circuit of said device, means for developing a voltage of carrier wave frequency to be modulated across the impedance between said grid and cathode and said reactance in series whereby currents of said frequency flow in said series circuit and a phase displaced voltage of said frequency is developed between said grid and cathode, means for varying the mutual conductance of said device at signal frequency to thereby correspondingly vary the phase of said last voltage, a circuit tuned to the frequency of saidlast voltage coupled between said grid and cathode, and an output circuit coupled to said first named tuned circuit.

8. In a ,wave length modulation system, an

electron discharge device having electrodes including a grid and a cathode, a reactance in the cathode circuit of said device, means for developing a voltage of carrier wave frequency to be modulated across the impedance between said grid and cathode and said reactance in series whereby currents of said frequency flow in said series circuit and a phase displaced voltage of said frequency is developed between said grid and cathode, means for varying the mutual conductance of said device at signal frequency to thereby correspondingly vary the phase of said last voltage, a circuit tuned to the frequency of said last voltage coupled between said grid and cathode, an electrondischarge tube system including a control grid, a cathode, and an output electrode, means coupling the control grid and cathode of said tube to said tuned circuit, and

an output circuit coupled to the output electrodes of said tube.

9. In a wave length modulator, an electron discharge device of high internal impedance having electrodes including a grid and a cathode, a reactance in the cathode circuit of saiddevice, means for developing a voltage of carrier wave frequency to be modulated across the impedance between said grid and cathode and said reactance in series whereby voltages of said frequency appear in said series circuit and a phase dis placed voltage of said frequency is developed between said grid and cathode, means for varying the mutual conductance of said device at signal frequency to thereby correspondingly vary the phase of said last voltage, a circuit tuned to the frequency of said last voltage coupled between said grid and cathode, a circuit tuned to the frequency of said last voltage coupled to said first named tuned circuit, and a frequency multiplier coupled to said last named tuned circuit.

10. ,In a wave length modulation system, an-

pedance between said grid and cathode and said reactance in series whereby currents of said frequency flow in said series circuit and a phase displaced voltage of said frequency is developed between said grid and cathode, means for varyingthe mutualconductance of said device at signal frequency to thereby correspondingly vary the phase of said last voltage, an electron discharge tube system of the pentode type hav ing electrodes including a control grid, a cathode, and an output electrode, an output circuit coupled to the output electrodes of said tube, and means for applying voltages developed between said grid and cathode of said device to the grid and cathode'of said tube of sufficient amplitude to overload said tube. v

11. In a wavelength modulation system, a pair of electron discharge devices each having electrodes including a grid and a cathode, a reactance in the cathode circuit of each of said devices, means for developing a voltage of carrier wave frequency to be modulated across the impedance between the grid and cathode of each device and the reactance in series therewith whereby currents of said frequency flow in said series circuits and phase displaced voltages of said frequency are developed between the grid and. cathode of each device, means for varying the mutual conductance of said devices at sig-, nal frequency to thereby correspondingly vary the phases of-said last voltages, and means for combining said last voltages in phase.

12. In a wave length modulator, an electron discharge device having electrodes including a grid and a cathode, a reactance, reactive at carrier wave frequency, in the cathode circuit of said device, means for developing a voltage of carrier wave frequency to be'modulated across the impedance between said grid and cathode and said reactance in series whereby currents of said carrier frequency flow in said series circuit and a voltage of said carrier frequency and of a phase which is displaced relative to the phase of said first voltage is developed between said grid and cathode, means for varying the mutual conductance of said device at signal frequency to there: by correspondingly vary the phase of said last voltage, and frequency multiplying means couphase varied voltage.

13. In a wave length modulation systemya pair of electron discharge devices each having electrodes including a grid and a cathode, an inductive reactance in the cathode circuit of one of said devices, a capacitive reactance in the cathode circuit of theother of said devices, means for developing a voltage of carrier wave frequency to be modulated across the impedance between the grid and cathode of each device and the reactance in'series therewith whereby currents of said frequency flow in said series circuits and phase displaced voltages of said carrier frequency are developed between the grid and cathode of each device, means for varying the mutual conductance of said devices at signal frequency to thereby correspondingly vary the phases of said lastvoltages, and means for combining'the said last voltages.

14. In a, wave length modulation system, a pair of electron discharge devices each having electrodes including a grid and a cathode, an inductive reactance in thecathode circuit ofone of said devices, a capacitive reactance in the cathode circuit of the other of said devices, means for developing in phase voltages of carrier wave 7 frequency to be modulatedacross the impedance between the grid and cathode of each device'and the reactance in series therewith whereby currents of said frequency flow in said series circuits and voltages oppositely displaced relative to the developed voltages of said carrier frequency ap pear between the grid and cathode of each device, means for varying the mutual conductance of said devices in phase opposed'relation at signal frequency to thereby correspondingly vary the phases of said last voltages, and electron discharge tube means coupled to the grid and cathode of each device for combining the said phase varied voltages in phase or in phase opposition.

CHARLES N. KIMBALL. 

